Compound dielectric material and process for manufacturing the same



Patented Mar. 4, 1947 COMPOUND DIELECTBIG- MATERIAL AND PRQCESS FOR MANUFACTURING THE SAME Kenneth H. Barnard, Andover, Jayson G. Balsbaugh, Newton, and Melville: 3?. Blacliwoodb. Andover, Massl, assigno'rs to Pacific Mills, Boston, Mass, a corporationo-f Massachusetts No Drawing. ApplicationfJ'anuary" 1'5; 194.2, Serial No. 426,886

5' Claims.

The present invention relates to a dielectric material and to a process for manufacturing the same.

It is an object of the invention to produce a dielectric material having a comparatively low power factor whereby the loss in power in an alternating current transmission line encased in the dielectric material is materially reduced. More specifically, it is an object of the invention to provide improvements in dielectric materials which. comprise laminated structures, constituent laminae possessing different dielectric prop1- erties- An examplev of such a material: is varnished cambric.

varnished cambric. or. varnished. cloth, in. the form of. tape, is widely. used. as. an insulating wrapping. for high tension alternating current cables and other electrical conductors... Such varnished clothis defective. because its absorpe tion of moisture. increases the. power. factor and means a substantialdissipation of energy in.the circuit. and a decrease in the amount of electrioal load that the circuit can carry. Efforts have been made to diminish these objections: by increasing the. electrical resistance'ofv the cloth as by purifying it. and removing electrically'co'm ductive impurities therefrom.

We have discovered that. in the alternating current carried by a conductor encasedin such an. insulating material, the power loss: can: be materially reduced, not by increasing the electrical resistance of the cloth, but, on the" contrary, by reducingv that electrical resistance, for example to a value less than 1/20000" of. the resistance in ohms of the dry cloth or fabric that has heretofore beenused. This is directly contrary to. present practice and trends. Thus we make the cloth an adequate conductor of electricity. For this purpose we load the cloth with conducting substances, such as finely divided or colloidal carbon black, graphite. salts; finely divided metal or starch.

lhe result of our treatment is for. all practical ing t'hemiuniformly and making them. fast there.-

in. Our invention provides an improvedprocess 3 improved process involves a pre-treatment of the cloth or fibrous material to create therein an affinity forthe finely-divided conducting material.

Thus, carbon black, which carries a negative charge dispersed in water, will normally be repelled by cotton. cloth. becausethe cloth. carries a similar charge. By treatment of the cloth with a. cationic agent of positive charge, however, an. affinity for the carbon black may be created;-

an example of. the practice of our process, the following may be given.

A light, closely-woven cotton cloth of combed yarn, thoroughly purified by boiling. bleaching and washing, was soaked in a 1% solution of Triton K-GO: (a quaternary ammonium c'om-.- pound) for five. minutes at 100 F.,. with constant agitation. Then the cloth was given a squeeze by a'- standard adjustment of. the squeeze rolls (the cloth probably holds 200% of its weight of water) and dried inathe oven. The clothw'as removed as soon as it. felt bone dry. The cloth was then immersed, with agitation, in a 12.5% aqueous dispersion of carbon black (using; polymeriz'ed organic salts of the alkyl-aryl' type; for example Daxad, as a dispersing agent) at 100 F. for five minutes. in open width under flowing water at F. until no more. carbon. black could be seen in the water running off. The cloth Was given a heavy squeeze and dried in the dryer. The-cloth" was then starche'd. by submerging it in open width" in 10 potato starch at F. or higher temperature, untilcom'pletely wet out.

An example of a procedure for making the starch bath is as follows: 100 lbs. of" potato starch and 10' lbs. of a wax emulsion was mixed in a" 100 gallon tub containing'cold water; with agitation. This mixturev was heated by'an' open steam. pipe and brought to a boil. Th'eltemperature was maintained until the potato starch was thoroughly. thickened. and. then the mix was transferred to av two-bowl mangle.-

The cloth treated with carbon black was drawn through the starch in the mangle and dried. on a sta-nda-rd tenter. frame. Subsequently, the starched cloth was sprinkled lightly with water and then. not friction calendered on both. sides, using. a pressure ofei) to "70 tons. The calendered The cloth was then washed 3 cloth was varnished with a black insulation varnish in the usual tower coating equipment.

The carbon black Was found to be well distributed through the cloth and tenaciously fixed therein. The electrical conductivity of the unvarnished cloth was greatly increased.

The negative charge of the carbon black is increased by absorption of the dispersing agent and the black should be so dispersed in this agent as to give it the optimum negative charge, so

' that it will be strongly attracted by the cationic substance on the cloth. The particle size of the carbon black should be sufficiently small to permit the black to enter the pores of the fibres and be absorbed therein. Furthermore, the electrical conductivity of the black is increased as particle size diminishes. The carbon black or other material should be sufficiently conductive to keep the power factor of the compound dielectric low and approximately equal to that of the varnish, even under conditions of high humidity and high temperature.

It is desirable to wash the cloth in water after the application of the carbon black and before drying in order to remove from the surface all carbon black that has not actually been absorbed onto or into the cloth. It is important that none of the carbon black find its way into the layers of insulating varnish which are .later applied.

Cationic materials generally, for example salts 7 of alkyl amines such as tetra alkyl quaternary ammonium halide, are effective in treating fibrous material in accordance with our invention. Aluminum acetate is representative of a class of cationic compounds of chemical structure different from that of salts of alkyl amines, which may be used.

In lieu of impregnating the cloth with carbon black or like conducting material, the cloth may be made electrically conducting by the proper addition of sizing materials such as starch, pro- ,viding such materials are added in quantities boiled, bleached cotton, Washed free of soaps and salts. V

It is advantageous to have the cloth as thin as possible, consistent with such desirable physical properties as tensile strength, stretch, tearing strength, etc. 7

The cloth thus obtained is a very good conductor of electricity as compared with the cloth previously used in varnished insulating tapes. We have obtained excellent results when the resistanceof the cloth is made as low as50 ohms for a i -mil thick conducting cloth measured with 2 in. diameter electrodes on opposite sides of the cloth. Good results,"however, may be obtained where the resistance value of the finished or impregnated cloth may be as much as 50,000 to 100, 000ohms or even 500,000 ohms. 500,000 ohms is about /zoooo of the resistance of the dry cloth used in varnished insulating tapes at the present time.

The contribution of the cloth to the percent power factor of the varnished tape of this inven tion may be expressed approximately by 100 RwC,

in which R is the resistance of the finished or im-' pregnated cloth in ohms per unit of area, a: is 21:-

. 4 r times the frequency in cycles per second of the alternating current and C is the capacitance in farads' per unit area'of the varnish film. This I value, 10.0 RwC, shouldbeas low as practicable, forexample not over 5 and preferablylgl, or less. The foregoing may be used to determine the allowable resistance of the'cloth for a given situation.

We have found that, while the electrical breakdown voltage of the dielectric embodying our invention is decreased in the order of about 20% by the use of the cloth of low resistance, as above described, the power factor is reduced to substantially that 01' the varnish itself which is roughly of the order of 5%. For this reason there is less heat generated in the dielectric and less loss of energy in the alternating high tension current transmitted by a conductor encased in our dielectric material than has heretofore been obterial has a longer life.

While we have described various features and embodiments of our invention in detail, we do not desire to be limitedto the exact details set ation within the scope of the appended claims. We claim: v -1. A dielectric material, adapted to be used as a tape for wrapping cables conducting high teninside cloth element consisting of cambric, electrically conductive finely divided carbon particles impregnating said cambric and absorbed and tenaciously fixed in the cambric by the electrical affinity of the cambric-and the carbonparticles and fast to washing, said carbon impregnated cambric havin at least 20,000'times less resist- 5O impregnating'said cambric and tenaciously fixed therein and' a coating of starch having a hard smooth surface on each side of said carbon impregnated cambric, said cloth' elementbeing substantially free froin other materials of electrical inuslating character, and a layer of insulating varnish on the surface of each starch coating. 7

r 3. A dielectric material, adaptedto be used as character, and a layer of insulatingvarnish on the surface of'each starch coating.

4. The-method of making a dielectric'material adapted to be used as a tape'for wrapping cables g conducting high tension alternating electric 'cur- -rents, which comprises impregnating a: cambric 5 with a solution of a cationic agent of'positive tained with an insulated conductor and the maforth, but the invention is susceptible of vari-/ sion'alternating electric currents, comprising an on each side of said carbonimpregnated cambric,

electrical charge, drying the cambric, impregnating the dried cambric with an aqueous dispersion of electrically conductive finely divided carbon particles whereby said particles are absorbed and tenaciously fixed in the cambric by the electrical affinity of the cambric and the carbon particles, washing said cambric free of loose carbon particles, applying a coating of starch to each side of said impregnated cambric, calendering said starched and carbon impregnated cambric to provide smooth hard surfaces on each side thereof, and applying a layer of insulating varnish to each of said surfaces.

5. A tape, adapted for insulating cables conducting high tension alternating electric current, comprising a textile fabric layer impregnated with electrically conductive finely divided carbon particles fast therein and coated with starch and being substantially free of other insulating substances, and layers of insulating varnish on the surfaces of said starch coating, the resistance in ohms, R, of the fabric being so related to the capacitance in farads, C, of the layers of insulating varnish that the contribution of the textile fabric layer to the percent power factor of the a tape, calculated as 100 RwC, (a: being 211' times 6 the frequency in cycles per second of the alternating current) does not exceed 5.

KENNETH H. BARNARD. JAYSON C. BALSBAUGH. MELVILLE J. BLACKWOOD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,102,129 Roseh Dec. 14, 1937 904,808 Rupley Nov. 24, 1908 2,092,489 Williams Sept. 7, 1937 2,028,189 Bowlby Jan/21, 1936 904,808 Rupley Nov. 24, 1908 2,234,252 Homan Mar. 11, 1941 2,338,480 Auxier Jan. 4, 1944 42,953 Liddell May 31, 1864 272,569 McNell Feb. 20, 1883 1,717,171 Schorger June 11, 1929 1,913,214 Schuhmann June 6, 1933 FOREIGN PATENTS Number Country Date 19,882 (Br.) Beaver 1914 

